This invention relates generally to determining position by electromagnetic radiation. More particularly, the invention relates to an improved system for using sensed position data to control vehicle barriers.
As the world becomes a more crowded and busy place, there are an increasing number of other vehicles on the road, on the rail, on the sea and in the air. Very early in the development of our roadway system, the traffic light was developed to control the flow of traffic at intersections. The earliest traffic lights were simply controlled by timers, each light was on for an allotted period of time within a cycle which repeated over and over. Some level of sophistication was added when the traffic patterns at a particular intersection were studied at the timers, no computer controlled, varying the timing of the traffic lights according to the predicted average traffic load for different times of the day. Yet it was recognized that the average load was frequently not the actual load for a given moment in time. Sensors in the road were developed and coupled to the traffic light controller so that the timing of the traffic light could be at least somewhat sensitive to the actual road conditions.
In addition, the land based and seagoing vehicles while they predominantly stay on their own mediums of transport sometimes will intersect each other. One example of this interaction is at a drawbridge. Because of the expense associated in building bridges which are high enough to accommodate the tallest of ships, the drawbridge has become a fixture on many coastal waterways. When a ship beyond a certain height must pass, the drawbridge operator must raise the drawbridge. When this happens traffic across the bridge will stop. As this is typically a highly manual operation, the occupants of the ship or the vehicles wishing to cross the bridge are subjected to long delays.
The Applicants propose an improved method of controlling crossings where two modes of conveyance intersect such as a drawbridge using position sensing. The Global Positioning System (GPS) is currently the most precise positioning system generally available to the general public and has significantly dropped in price in recent years. More and more vehicles come equipped from the factory with GPS and this trend is expected to continue. The GPS comprises a network of 24 satellites orbiting the earth. Each satellite transmits a ranging signal modulated on a 1.575 Ghz carrier. By monitoring the signal from a plurality of satellites, a GPS receiver can determine its position, i.e. latitude, longitude and altitude, to an accuracy of about 15 meters. In general, this degree of accuracy would be attained if signals from three or four of the GPS satellites were received. More accurate GPS signals are available to the military. Differential GPS, also available to the public, is more accurate than standard GPS, but requires an additional land based transmitter and special permission from the government.
Many of the uses for GPS-based systems known to the Applicants are in the realm of mapping or collision avoidance applications. Notably one such GPS-based system is taught by "Traffic Alert and Collision Avoidance Coding System", U.S. Pat. No. 5,636,123 to Rich et al. In the Rich system, the airspace is divided up into a grid of volume elements. A collision avoidance signal is transmitted wherein the carrier signal is modulated by a psuedonoise code which is function of the volume element in which the aircraft is located. Each aircraft only tracks collision avoidance signals from vehicles in its own and immediate surrounding cells. Based on the calculated paths of the aircraft, a warning of an impending collision can be provided to the pilot.
The Applicants have proposed an improved tracking and collision avoidance system in "Time Multiplexed Global Positioning System Cell Location Beam System" Ser. No. 09/239,335, filed the same day as the present application, is commonly assigned and is hereby incorporated by reference. Although the invention described in the incorporated application does not address the problems of controlling traffic lights, it does share an overall cell structure with the preferred embodiment of the present invention.
This invention solves these and other important problems.